Package for millimeter wave molecular clock

ABSTRACT

In a described example, an apparatus includes: a package substrate having a device side surface and a board side surface opposite the device side surface; a physics cell mounted on the device side surface having a first end and a second end; a first opening extending through the package substrate and lined with a conductor, aligned with the first end; a second opening extending through the package substrate and lined with the conductor, aligned with the second end; a millimeter wave transmitter module on the board side, having a millimeter wave transfer structure including a transmission line coupled to an antenna aligned with the first opening; and a millimeter wave receiver module mounted on the board side surface of the package substrate and having a millimeter wave transfer structure including a transmission line coupled to an antenna for receiving millimeter wave signals, aligned with the second opening.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation to U.S. patent application Ser. No.17/246,300, filed Apr. 30, 2021, which claims the benefit of andpriority to U.S. Provisional Application No. 63/133,231, filed Dec. 31,2020 which is hereby incorporated by reference herein in its entirety.

TECHNICAL FIELD

This relates generally to packaging electronic devices, and moreparticularly to packaging semiconductor devices and a physics cell in amillimeter wave molecular clock module.

BACKGROUND

Increasingly, highly accurate and stable clocks are needed forapplications such as navigation and ranging, autonomous vehicle control,and location services on devices including portable and handhelddevices. A physics cell includes a dipolar gas that exhibits quantumrotational transitions. The physics cell can be used to form a referencesignal for a clock. The physics cell includes the dipolar gas in asealed gas chamber. Electromagnetic energy in the form of a millimeterwave frequency signal, in an example a signal of over 100 GHz, istransmitted to the physics cell by a millimeter wave transmitter, andremaining signal energy is received from the physics cell by amillimeter wave receiver. The dipolar gas in the physics cell absorbsmaximum energy at quantized frequencies which cause a molecularrotational transition. The transmitted signal frequency can be adjusteduntil a molecular rotational transition is detected, the energyabsorption of the molecular rotation transition can be detected as adrop in amplitude of the remaining energy received from the physicscell. The transmitted signal frequency can be locked to the quantizedmolecular rotation frequency. Because the quantized molecular rotationfrequency is highly stable over temperature and over time, the lockedtransmitted frequency signal can be used as a stable reference forgenerating a clock signal or a reference frequency signal. A costeffective, robust and reliable package for the millimeter wave molecularclock module is needed.

SUMMARY

In a described example, an apparatus includes: a package substratehaving a device side surface and a board side surface opposite thedevice side surface; a physics cell mounted on the device side surfaceof the package substrate and having a first end and a second end; afirst opening extending through the package substrate and lined with aconductor, the first opening aligned with the first end of the physicscell; a second opening extending through the package substrate and linedwith the conductor, the second opening aligned with the second end ofthe physics cell; a millimeter wave transmitter module mounted on theboard side surface of the package substrate, having a millimeter wavetransfer structure including a transmission line e coupled to an antennaconfigured to transmit millimeter wave signals aligned with the firstopening; and a millimeter wave receiver module mounted on the board sidesurface of the package substrate and having a millimeter wave transferstructure including a transmission line coupled to an antenna forreceiving millimeter wave signals from the physics cell, and alignedwith the second opening.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an arrangement for a molecular clocksystem.

FIG. 2A is a cross section of a package arrangement for millimeter wavemolecular clock module, FIG. 2B illustrates in a cross sectional view aportion of a molecular clock system including the package of FIG. 2A ona printed circuit board.

FIG. 3A illustrates in a cross sectional view a physics cell for usewith an arrangement; FIG. 3B illustrates in an alternative arrangement aphysics cell; FIG. 3C is a plan view of a physics cell for use with thearrangements.

FIG. 4A illustrates in a graph the absorption frequencies of an OCS gascell having quantum rotational transitions; FIG. 4B illustrates in agraph an example of absorption of a signal at a quantum frequency.

FIGS. 5A-5B illustrate in a plan view and a cross section, respectively,a package substrate for use with the arrangements.

FIG. 6 illustrates, in a cross sectional view, a portion of anarrangement including a physics cell mounted on a package substrate.

FIGS. 7A-7D illustrate, in a cross sectional view, a close up top view,and in further cross sectional views, portions of a millimeter wavemodule for use with the arrangements.

FIG. 8 illustrates, in a projection view, a physics cell for use withthe arrangements.

FIGS. 9A-9B illustrate, in a side view and an end view, a physics cell,a package substrate and millimeter wave modules in a molecular clockmodule package of the arrangements.

FIG. 10 illustrates, in a flow diagram, a method arrangement.

DETAILED DESCRIPTION

Corresponding numerals and symbols in the different figures generallyrefer to corresponding parts, unless otherwise indicated. The figuresare not necessarily drawn to scale.

Elements are described herein as “coupled.” As used herein, the term“coupled” includes elements that are directly connected, and elementsthat are electrically connected even with intervening elements or wiresare coupled.

The term “semiconductor die” is used herein. As used herein, asemiconductor die can be a discrete semiconductor device such as abipolar transistor, a few discrete devices such as a pair of power FETswitches fabricated together on a single semiconductor die, or asemiconductor die can be an integrated circuit with multiplesemiconductor devices such as the multiple capacitors in an A/Dconverter. The semiconductor die can include passive devices such asresistors, inductors, filters, or active devices such as transistors.The semiconductor die can be an integrated circuit with hundreds orthousands of transistors coupled to form a functional circuit, forexample a microprocessor or memory device. The semiconductor die can bea passive device such as a sensor, example sensors include photocells,transducers, and charge coupled devices (CCDs), or can be a microelectro-mechanical system (MEMS) device, such as a digital micromirrordevice (DMD).

The term “packaged electronic device” is used herein. A packagedelectronic device has at least one semiconductor die and has a packagebody that protects and covers the semiconductor device die. In somearrangements, multiple semiconductor dies can be packaged together. Forexample, a power metal oxide semiconductor (MOS) field effect transistor(FET) semiconductor device die and a logic semiconductor device die(such as a gate driver die or controller device die) can be packagedtogether to from a single packaged electronic device. Additionalcomponents such as physics cells, and other passives can be included inthe packaged electronic device. The semiconductor device die can bemounted to a substrate that provides conductive leads, a portion of theconductive leads form the terminals for the packaged electronic device.The semiconductor die can be mounted to the substrate with an activedevice surface facing away from the substrate and a backside surfacefacing and mounted to the substrate. Alternatively, the semiconductordevice die can be flip-chip mounted with the active surface facing thesubstrate surface, and the semiconductor device die mounted to the leadsof the substrate by conductive columns or solder balls. The packagedelectronic device can have a package body formed by a thermoset epoxyresin in a molding process, or by the use of epoxy, plastics, or resinsthat are liquid at room temperature and are subsequently cured. Thepackage body may provide a hermetic package for the packaged electronicdevice. The package body may be formed in a mold using an encapsulationprocess, however, a portion of the leads of the substrate are notcovered during encapsulation, these exposed lead portions provide theexposed terminals for the packaged electronic device. The package bodycan be a metal shell or cover that protects a device. Protective fillerssuch as low modulus material can be used to protect the device.

The term “package substrate” is used herein. A package substrate is asubstrate arranged to receive a semiconductor die or device and tosupport the semiconductor die or device in a completed semiconductorpackage. Package substrates include conductive lead frames, which can beformed from copper, aluminum, steel and alloys such as Alloy 42 andcopper alloy. The lead frames can include a die pad for mounting thesemiconductor die, and conductive leads arranged proximate to the diepad for coupling to bond pads on the semiconductor die using wire bonds,ribbon bonds, or other conductors. The lead frames can be provided instrips or arrays. Dies can be placed on the strips or arrays, the diesplaced on a die pad for each packaged device, and die attach or dieadhesive can be used to mount the dies to the lead frame die pads. Wirebonds can couple bond pads on the semiconductor dies to the leads of thelead frames. After the wire bonds are in place, a portion of thesubstrate, the die, and at least a portion of the die pad can be coveredwith a protective material such as a mold compound.

Alternative package substrates include pre-molded lead frames (PMLF) andmolded interconnect substrates (MIS) for receiving semiconductor dies.In one example, a MIS package substrate is referred to as a “routablelead frame” or RLF. These substrates can include dielectrics such asliquid crystal polymer (LCP) or mold compound and can include one ormore layers of conductive portions in the dielectrics. The lead framescan include plated, stamped and partially etched lead frames, in apartially etched lead frame, two levels of metal can be formed byetching a pattern from one side of the metal lead frame, and then fromthe other side, to form full thickness and partial thickness portions,and in some areas, all of the metal can be etched to form openingsthrough the partial etch lead frames. Repeated plating and patterningcan form multiple layers of conductors spaced by dielectrics, andconductive vias connecting the conductor layers through the dielectrics,the dielectrics can be mold compound. The package substrate can also betape-based and film-based substrates carrying conductors; ceramicsubstrates, laminate substrates with multiple layers of conductors andinsulator layers; and printed circuit board substrates of ceramic,fiberglass or resin, or glass reinforced epoxy substrates such as FR4.In an example arrangement, a package substrate of FR4 is used to mount aphysics cell on a first surface, and millimeter wave transmitter andreceiver modules are mounted on a second surface. Additional integratedcircuits can be mounted to the package substrate.

The term “millimeter wave signal” is used herein. Millimeter wavesignals have a frequency of between 30 GHz and 300 GHz. In examplearrangements, frequencies of greater than 100 GHz, for example about 121GHz or 133 GHz, are used to stimulate a physics cell. In thearrangements, a millimeter wave transmitter module is used, this moduleincludes an integrated circuit that transmits millimeter wave signals. Amillimeter wave receiver module is used in the arrangements, this moduleincludes an integrated circuit that receives millimeter wave signals. Inthe arrangements, a frequency corresponding to a molecular rotationtransition frequency is referred to. In an example, the frequency for adipolar gas is 121.62 GHz. The molecular rotation frequencies for adipolar gas are physical constants. However, in the arrangements asignal transmitted at a frequency transitions different materials andtraverses dielectric materials and air. Some variation in the observedor transmitted frequency can occur, due to the materials used in thearrangements, so that a normal variance of +/−10% may be measured. Inthis description, when a particular frequency is described, a varianceof +/−10% is included, so that a molecular rotational frequency of121.62 GHz means 121.62 GHz +/−10%; and similar variance is included inother described frequencies.

The term “physics cell” is used herein. A physics cell is a cell whichexhibits a physical constant. In the arrangements, a dipolar gas physicscell has quantized molecular rotation. In response to RF signals appliedat one of a series of discrete frequencies, the molecules in the dipolargas physics cell absorb the energy and transition from a firstrotational state to a second rotational state. When energy at otherfrequencies away from the discrete quantized frequencies are applied tothe physics cell, the molecules will not transition and the energy isnot absorbed. Because the energy is almost completely absorbed by thecell at certain frequencies that correspond to quantized molecularrotational transitions, a receiver can detect when the transmittedfrequency is at a quantization frequency for the cell, and a system canlock to the transmitted frequency. The quantized rotational frequenciescorrespond to a physical constant and are highly stable over time andover temperature, so that a stable constant frequency reference can beobtained by use of the physics cell.

In the arrangements, a package substrate has a physics cell mounted on adevice side surface and millimeter wave transmitter module and amillimeter wave receiver module mounted on an opposing board sidesurface. A portion of the package substrate and the physics cell can becovered with a shell, dielectric, shield or lid to protect the physicscell. The millimeter wave transmitter module and the millimeter wavereceiver module include millimeter wave integrated circuits mounted onhigh performance, high frequency circuit board substrates. Themillimeter wave transmitter module and millimeter wave receiver moduleare mounted on a board side surface of the package substrate oppositethe device side surface. Openings extending through the packagesubstrate act as millimeter wave coupling elements, and operate asvertical waveguides. A first opening in the package substrate isarranged to receive a gigahertz frequency interrogation signal from themillimeter wave transmitter module. The first opening is lined with aconductor and forms a vertical waveguide, in an example the waveguide issized to form a rectangular waveguide. The dipolar gas cell has an iristo receive energy at one end of the gas chamber, which receives theinterrogation signal with high insertion loss and low reflectivity. Atan opposite end of the dipolar gas cell, a second iris that transmitsthe remaining signal energy is coupled to a second rectangular openingin the package substrate, this second opening is also a vertical openinglined with a conductor and sized to form a rectangular waveguide. Themillimeter wave receiver module is also provided on a high performancecircuit board mounted on the board side surface of the package substrateand includes a signal probe on the end of a coplanar waveguide coupledto the second opening in the package substrate. The package substrateis, in one example, a fire resistant glass fiber reinforced epoxy, suchas FR4. Other substrate materials can be used for the package substrateincluding ceramic and BT resin substrate materials. The packagesubstrate is not required to be a high performance substrate, as themillimeter wave signals traverse openings extending through the packagesubstrate, and the millimeter wave signals are not coupled to any traceswithin the dielectric materials of the package substrate. Use of lowfrequency substrate materials for the package substrate reduces cost forthe package substrate and the molecular clock system. The arrangementsprovide a cost effective module for use in a molecular clock system,which has been shown in simulations to be highly stable over a widetemperature range.

FIG. 1 illustrates in a block diagram a molecular clock system 100.Module 101, which is a packaged molecular clock module of thearrangements, includes a physics cell 103, which in an examplearrangement includes a dipolar gas in a sealed chamber, the dipolar gascan be an OCS (oxygen carbon sulfide) or carbonylsulfide, gas. Thedipolar gas exhibits quantum molecular rotation so that when a signal ofa frequency at a discrete rotational frequency is used to stimulate thegas in the physics cell, a quantum molecular rotation occurs that can beobserved. Other dipolar gasses can be used, including the OCS, watervapor, hydrogen cyanide (HCN), hydrogen chloride (HCL), and acetonitrile(CH₃CN). The transmitted signal can then be used as a referencefrequency, and the system can be locked to the quantized molecularrotation frequency of the physics cell in a control loop. In an examplearrangement the frequency used is about 121.62 GHz, a molecularrotational frequency for OCS gas, but other transitions at other quantumrotation frequencies may be used. For example a second quantum rotationfrequency for the OCS cell is at about 133.78 GHz.

A transmitter die 105 on a transmitter module is used to provide amillimeter wave interrogation signal to the physics cell 103. A receiverdie 107 on a receiver module is coupled to receive the millimeter wavesignal from the physics cell 103. The clock module 101 is coupled to ananalog-to-digital converter (ADC) 109 which samples the analog signalfrom the clock module 101 and outputs a digital signal corresponding tothe analog signal, indicating the amplitude or magnitude of the analogsignal. A microcontroller 112 performs a control loop using, forexample, proportional integral derivative (PID) control. Themicrocontroller 112 tunes a clock generated by controlling a fractionalN circuit 116. The fractional N circuit 116 includes a PLL and a signalsynthesizer and provides two outputs, one is a 10 MHz clock signal to beused as an output signal for system 100, which is the stable clocksignal generated by the system, the other is a 320 MHz signal to be usedin a feedback loop. An oscillator 114 provides a raw clock, for examplea 630 MHz clock, to the fractional-N circuit 116, other raw clock inputsignals can be used. In an example this oscillator can be a bulkacoustic wave (BAW) device, other oscillators can be used. Thefractional N circuit 116 has control inputs that allow themicrocontroller 112 to adjust an output clock generated from the rawclock signal, and thus tune and lock the control loop. In this examplearrangement, a second fractional N circuit 118 is shown receiving the320 MHz clock and outputting a reference signal of 10.135 GHz, inanother example an output can be 11.1487 GHz. The reference signal fromthe fractional N device 119 is input to module 101 and is used by thetransmitter die 105 to generate the interrogation signal, for example of121.62 GHz, by an up multiplier “×12”. While two Frac-N devices areshown in the example of FIG. 1 , in other arrangements a single Frac-Ndevice can be used. In some arrangements the BAW oscillator 114 can bewithin the clock module 101.

In operation, the system 100 outputs a stable reference clock signal of10 MHz using the physics cell 103 to develop a stable frequencyreference used for generating the clock signal. The transmitter die 105sends an interrogation signal near an expected quantum rotationalfrequency to the physics cell 103. The remaining signal output from thephysics cell 103 is received by the millimeter wave receiver die 107,which outputs the power or amplitude of the remaining signal, and theoutput signal is filtered and conditioned and output to the ADC 109. Adigital signal corresponding to the magnitude of the output signal from101 is then input to microcontroller 112 for use in the PID controltracking loop. When the interrogation millimeter wave signal from thetransmitter die 105 matches the quantum rotational frequency of thedipolar gas physics cell, the dipolar gas physics cell will absorb mostof the energy, and the output signal into the millimeter receiver die107 will be at an amplitude minimum. The microcontroller 112 can adjustthe frequency output by fractional N circuit 116 until the minimumamplitude corresponding to the quantized rotational transition isobserved, and when that is achieved, the microcontroller 112 can lockthe circuit to the reference frequency of physics cell 103.

FIG. 2A illustrates in a cross sectional view a packaged millimeter waveclock module 101 of the arrangements. In FIG. 2A a physics cell 103 ismounted on a device side surface of a package substrate 131. A gaschamber 111 is shown in the cross section with two ends exposed, the gaschamber is continuous in the physics cell 103 which can be formed in asemiconductor substrate. The gas chamber 111 is sealed by glass bondedto the semiconductor substrate on two surfaces, and is lined on allsides of the gas chamber with a conductor such as gold. Other conductorsof noble metals can be used to line the gas chamber. Other conductorscan be used, and may be coated with anti-corrosion layers to ensure thegas in the chamber does not react with the conductors. Copper withadditional coatings can be used. A millimeter wave transmitter module113 is mounted on a board side surface of the package substrate 131, andtransmitter die 105 is mounted on the module 113. A millimeter wavecoupling structure 133 is formed between the millimeter wave transmittermodule 113 and the physic cell 103. A millimeter wave receiver module115 is mounted on the board side surface of the package substrate 131,and carries millimeter wave receiver integrated circuit 107. Amillimeter wave receiver structure 135 couples the physics cell 103 tothe millimeter wave receiver module 115. Terminals 161 for the package101 are formed on the board side surface of the package substrate 131,so that the package 101 can be mounted to a system board. The terminals161 can be ball grid array terminals such as solder balls, solderpillars, copper pillar pumps, copper columns, solder columns, stud bumpsor other ball grid array connections. A lid 151 covers the physics cell103 and the device side surface of the package substrate 131, and can ametal, for example. The lid 151 can form an RF shield. The lid 151 canbe of other materials that will protect the physics cell, and can bereplaced by an overmolded dielectric such as a mold compound, resin,epoxy, or plastic. Low modulus material can be used to reduce stress onthe physics cell 103 in addition to a metal lid 151 or overmoldedmaterial used to cover the physics cell 103.

FIG. 2B illustrates, in another cross section, a portion of a system 100mounted on a system printed circuit board (PCB) 120. The componentsexternal to the packaged clock module 101 are shown mounted to the PCB120, for example the fractional N circuits 116, 119, the microcontroller112, a power circuit 117. The packaged clock module 101 is shown in thecross section and includes the physics cell 103, a package substrate131, the millimeter wave transmitter module 113 with transmitter die105, and the millimeter wave receiver module 115 with receiver die 107.Terminals 161 mount the packaged module 101 to the PCB 120.

In FIGS. 2A-2B, the package substrate 131, which can be an FR-4substrate, supports the physics cell 103 and the millimeter wavetransmitter and receiver modules 113, 115. The package substrate 131 hasa transmitter coupling structure 133 and a receiver coupling structure135. These two coupling structures are formed using openings in thepackage substrate 131 that are lined with a conductor, such as a platedcopper, and are shaped to form waveguides that provide a verticalmillimeter signal connection between the transmitter module 113 and oneend of the physics cell 103, and another vertical connection between theopposite end of the physics cell 103 and the receiver module 115. Byusing inexpensive substrate materials and arranging the openings to actas rectangular waveguides, the package substrate 131 provides a costeffective package solution for the clock module 101. Further, in thearrangements the clock module 101 with an FR4 package substrate has beenshown to be extremely stable over temperature changes. Packaged clockmodule 101 provides a set of devices in one component for use in makinga clock system, increasing integration and ease of use, and reducingarea needed on the system board.

FIG. 3A illustrates in a cross sectional view the physics cell 103. Asubstrate 301 includes a gas cell 111 with a tubular, polygonal orcircular cross section that is lined with a conductor (not shown forclarity of illustration). In one example process, two wafers are used toform substrate 301. Each wafer has glass substrate bonded to it in awafer bonding process. One half of the chamber is etched as a cavitypattern into each wafer. The cavities are metallized with the conductor,for example, gold. The two wafers are placed face to face, the gas isintroduced, and an AuIn wafer bond process is used to complete thechambers, trapping the gas in the chambers. The individual physics cellson the wafers are separated from the wafers in a singulation process. InFIG. 3A, the two die portions 3011, 3012 are bonded at the dashed line3014 to form the substrate 301. Glass is used, for example to form a top305 and bottom 307 layer. A low pressure dipolar gas, in thearrangements OCS gas, is contained in the gas chamber 111 of physicscell 103. Iris openings 321, 323 are formed at opposite ends of thephysics cell to receive and transmit the millimeter wave interrogationsignals. These openings are made in the conductor layer (not shown) toallow the millimeter wave signals to enter and exit the gas chamber 111.

A process for forming a sealed gas cell with molecular rotation isdescribed in U.S. Pat. No. 9,529,334, titled “ROTATIONAL TRANSITIONBASED CLOCK, ROTATIONAL SPECTROSCOPY CELL, AND METHOD OF MAKING SAME”,which is hereby incorporated herein by reference in its entirety. Thephysics cell 103 can be made using semiconductor process technology. Afirst die portion 3011 and a second die portion 3012 are formed usingsilicon etch on two wafers to make a cavity into a silicon substrate,for example, and the two portions are bonded face to face using AuInwafer bonding (indicated by the dashed line 3014) to form gas chamber111. Because wet etching of silicon using an anisotropic etch, forexample TMAH wet etch, produces a trench with sloping sides, thepolygonal shape shown in FIG. 3A for the gas chamber 111 is obtained. Ifa different silicon etch process is used, for example a dry reactive ionetch (DRIE) process, the trenches in the two dies may have more or lessvertical sidewalls and the gas chamber can have a square or rectangularshape, such as is shown for an alternative shaped physics cell 331 asshown in a cross section in FIG. 3B, when the two dies 3311 and 3312 arebonded face to face as indicated by the dashed line 3314. A rectangulargas cell 311 is then formed. The gas chambers 111, 311 shown in FIGS. 3Aand 3B in cross section with two ends exposed, but are a continuouschamber plated with a conductor. Two openings are formed in theconductor at the two ends of the gas chamber. The gas chamber isdesigned with dimensions that are determined to provide efficienttransmission of the millimeter wave signals from a first end of the gaschamber to a second end of the gas chamber.

The gas chamber is shaped and sized to act as a waveguide. In anexample, when the OCS gas is used and the selected resonant frequency isabout 121 GHz as is further described below, the gas chamber 111 can bedimensioned similar to rectangular waveguide WR8, which has a band ofoperation from 70 GHz to 140 GHz, and has dimensions of about 2millimeters×1 millimeter in cross section. By sizing the gas chamber 111or 311 appropriately for the frequency signals being used, the mode ofpropagation can be a single transverse electric (TE) mode, making thetransfer of energy through the gas chamber most efficient. For example,when the frequency of the signals being applied to the gas chamber areabout 121 GHz, a transverse electric propagation mode of TE10 isdesired. Further the transmission through the gas chamber should beconfined to a single transverse electric mode (“monomode”) to avoidlosses by mode conversion. By sizing the gas chamber appropriately, andby designing the dimensions of other elements of the system to makeefficient transmission at transmitted frequencies, a system with highinsertion loss and low reflective loss from the transmitting millimeterwave integrated circuit to the receiving millimeter wave integratedcircuit is achieved. The length of the gas chamber 111 affects themagnitude or “depth” of the absorption response. When the quantizedfrequency signal is applied to the physics cell, the dipolar gas willabsorb the energy and transition from a first rotational state to asecond rotational state. When other frequencies are applied, the signalwill not be absorbed by the gas chamber and most of the energy will bereceived by the receiver. To increase the signal to noise ratio (SNR)and the quality of the response, the gas chamber 1 should be made aslong as is practicable for a given package substrate area. In theexample arrangements illustrated here, a serpentine shape is used forthe gas chamber to lengthen it on the substrate. In an alternativearrangement, the gas chamber can be made a “U” shape. In examplearrangements, the length can vary from end to end from about 10millimeters to 150 millimeters for an OCS gas cell, with a longer cellproviding better SNR than a shorter cell.

As shown in FIG. 3A, the gas chamber 111 has an “iris” opening 321, 323at each end where the conductive liner is opened to allow signals toenter, and exit, the gas chamber. A metal pattern on the glass surface(not shown, see FIG. 3C) can provide a shield surrounding the irisopening at each end.

FIG. 3C illustrates the physics cell 103 in a plan view. Substrate 301is shown with the tubular gas chamber 111 extending from a first coupler341 at a first end and having a second coupler 343 at a second end. Thecouplers are positioned around the iris openings 321, 323 in theconductor lining the tubular gas chamber 111 and are configured toreceive, and transmit, millimeter wave energy into the dipolar gaswithin the gas chamber 111.

FIG. 4A illustrates the absorption frequency spectrum for OCS gas over arange of GHz frequencies from less than 60 GHz to above 180 GHz. Themolecular rotational frequencies are quantized at discrete frequencies,about 12 GHz apart from one another. The absorption is plotted on thevertical axis while frequency is plotted on the horizontal axis.Referring to FIG. 1 , as signal absorption increases in the physicscell, the remaining signal that will be output to the receiver by thephysics cell decreases, further as can be seen in the graph in FIG. 4 ,the absorption is quantized. By applying an interrogation signal nearone of the quantum rotational levels, and by then tuning theinterrogation signal, the quantum rotational frequency can be used tolock a signal at a known frequency.

Use of the dipolar gas cell has several advantages. Unlike some otheratomic clock cells which are stimulated with optical energy and requiretemperature control to remain stable, such as a precise heating element,the dipolar gas cell is stable over temperature, reducing the need totightly control the cell temperature using heaters or other addedcircuitry. Unlike optically stimulated clock cells, in the arrangementsno optical transducers are needed. In example arrangements, a quantumfrequency 401 of about 121.62 GHz was used, although it can be seen fromexamining FIG. 4A that other quantum frequencies may also be used. Auseful alternative would be the next adjacent higher quantum frequency,of 133.78 GHz. Another useful alternative would be the next lowerquantum frequency, of 109.46 GHz. Because the wavelength of a 121.62 GHzsignal, in air, is about 2 millimeters, this frequency is particularlyuseful in the arrangements as the available dimensions of the variouscomponents relative to the wavelength make design of an efficient andcost effective transmit and receive path feasible. Note that thewavelength of the millimeter wave signals in a low-k dielectric material(such as in a high performance substrate) will be less than in air, forexample, 900 microns or less at 121.62 GHz.

There are design trade-offs that need to be made in determining thedimensions of the gas chamber, the size and shapes of the openings inthe package substrate, and the design of the millimeter wave transmittermodule, and the corresponding millimeter wave receiver module. As can beseen in FIG. 4A, as frequency increases, the magnitude of the response(amplitude or signal strength) at the quantized rotation frequenciesalso increases. Detecting the absorption frequency is clearly enhancedby a stronger response. However, as frequency is increased above 100 GHzand even further such as 180 GHz, the availability of cost effectivetransmitter and receiver integrated circuits is reduced. If a frequencyabove the Fmax cut off limit for CMOS technology integrated circuits isselected, the cost of the system will increase, as the integratedcircuits will need to be of more expensive technologies, such as GaAs,which can operate at higher frequency. The dimensions of the variousfeatures of the arrangements are also related to the wavelengths of thesignals used. A signal of 121 GHz has a wavelength, in air, of about2.47 millimeters. The size of the openings in the package substrate, thecross sectional dimensions of the gas chamber, and the design of themillimeter wave transmitter module and the millimeter wave receivermodule, are all impacted by the frequency and the wavelength of thesignals. Simulations including the various elements can be used toinsure the system has high insertion loss between the transmittingintegrated circuit and the receiving integrated circuit, and lowreflective loss, for the frequencies selected. When a 121 GHz signal isused, the gas chamber and the openings in the package substrate can bearranged similar in dimensions to a WR8 rectangular waveguide, which hasa useful frequency bandwidth from about 90 GHz to about 140 GHz. Thiswaveguide has a width of about 2 millimeters and a length of about 1millimeter in cross section, which are useful sizes for the packagesubstrate and physics cell of the arrangements. Higher frequency signalswill have correspondingly smaller wavelengths, in contrast lowerfrequency signals will have larger wavelengths. In selecting the dipolargas, the rotational frequency to be used, and dimensions for the variouselements of the arrangements, simulations can be performed to ensure themodule will have the desired performance. The selection of themillimeter wave transmitter and receiver integrated circuits is alsodetermined, in part, by the frequency of the signals being used.

FIG. 4B illustrates in a graph an example frequency response showing anamplitude minimum for an OCS dipolar gas cell in response to a quantizedrotational frequency signal. The rotation frequency Fr causes themolecules in the gas cell to absorb the energy, using the signal energyto transition from a first rotational state to a second rotationalstate, so that the remaining energy output from the gas cell is at aminimum; as shown in FIG. 4B, the magnitude of the remaining signaldrops sharply at the rotational frequency Fr. The millimeter wavereceiver and processor (see FIG. 1 ) can then be used to detect therotational frequency, and lock the frequency to the rotational frequencyof the gas in the physics cell. A highly stable clock signal can begenerated using the transmitted signal as a reference.

While OCS is a useful dipolar gas for the example arrangements, othergasses that exhibit quantized rotation can be used. The choice of thegas can be determined considering several factors, including thequantized spectrum frequencies available, whether the gas is corrosiveor reacts with metal such as the conductor liner of the gas cell, whichmay reduce the volume of available molecules over time (for examplewater vapor can be consumed in an oxidation reaction with copper);whether the gas is a stable molecule over time; whether the gas is safeto humans or animals if it should escape the gas cell into theenvironment; and whether the gas is safe to use in production of the gascells. Availability and cost of the gas are also considered. Thedimensions of the openings in the substrate, the cross sectional area ofthe gas chamber, the length of the gas chamber and other dimensions canalso be impacted by the choice of the gas, because the rotationalfrequency will change. As described above, the choice of the rotationalfrequency used changes the wavelength of the signals and impacts thedimensions of the various elements.

FIG. 5A illustrates, in a plan view, an example package substrate 131for use in an arrangement. Openings 141, 143 are used to form verticalwaveguide structures, along with additional components described below.Conductor material 145, which can be a copper plated material, goldplated material, or other conductor, lines the openings 141, 143.Substrate 131 may have additional conductors and traces formed to routesignals, or power, to the millimeter wave modules, as shown in FIG. 1above. None of the millimeter wave signals are routed in traces formedin the package substrate 131, making the use of a lower cost, lowfrequency substrate material possible. Instead the millimeter wavesignals traverse the openings 141, 143 which act as millimeter wavecouplers, or as vertical waveguides. In an example arrangement, thefirst opening and the second opening are rectangular in cross sectionand have a width and length of approximately 1 millimeters andapproximately 2 millimeters, respectively, in an example arrangementwhen a transmitted frequency signal is about 121.62 GHz, a quantizedrotational frequency for OCS gas. If the frequencies used or the dipolargas selected change, then other opening sizes can be used to makeefficient millimeter wave transmission to and from the physics cell.

FIG. 5B is a cross sectional view of the package substrate 131 takenalong the line B-B′ in FIG. 5A, showing the openings 141, 143 extendingthrough the package substrate 131, and having conductor material 145lining the openings. The openings and conductor in the package substratecan be formed using printed circuit board fabrication techniques, bylaser drilling, etch or stamping the openings and by plating theconductor liner 145. Note that the plan view of FIG. 5A shows theopenings 141, 143 positioned to correspond to the ends of the gaschamber of the physics cell 103. When a different shaped physics cell isused in an alternative arrangements, the positions of the openings 141,143 will be moved in correspondence with the different shaped tubulargas chamber. The dimensions of the package substrate will be determined,in part, by the size of the physics cell. The physics cell will havebetter SNR performance as longer lengths of the gas chamber are used, soit may range from 10 millimeters to 150 millimeters in length. Thepackage substrate to support these physics cells can be from 12×12millimeters squared, to about 25×25 millimeters squared. The packagesubstrate can have a thickness similar to substrates used forsemiconductor devices, for example, from 1 to 2.5 millimeters. Thickersubstrates can also be used.

FIG. 6 illustrates in a cross-sectional view a partial assembly step ofmodule 101. The package substrate 131 has the physics cell 103 mountedon a device side surface, and a cover 151 is shown protecting thephysics cell 103. Openings 141, 143 are shown positioned incorrespondence with the first and second ends of the gas chamber 111.Openings 141, 143 will be used to transmit and receive the millimeterwave signals to the physics cell 103.

FIG. 7A illustrates, in a cross sectional view, a millimeter wavetransmitter module 113 for use with the arrangements. In FIG. 7A, atransmitter die 105 is shown on a high performance, high frequencysubstrate 703. In an example, a millimeter wave compatible highfrequency substrate from Shinko was used with four levels of metalconductors in a dielectric material. Substrates with more or fewerconductor levels can be used. Solder balls or solder bump connections711 are shown placed on the die side surface of substrate 703. A topground conductor layer 720 overlies the surface of substrate 703. Acoplanar waveguide structure 723 is formed in a metal layer of thesubstrate 703 below the top ground layer 720 and is coupled to thetransmitter die 105 by a conductive via or trace. This coplanarwaveguide structure 723 is positioned with an antenna end 721 placed incorrespondence with an opening 141 in the package substrate 131 (seeFIG. 2 , for example) and will transmit signals into that opening. Abottom ground plane 726 is formed by a conductor 727 that is below thecoplanar waveguide 723. The bottom ground plane is beneath the coplanarwaveguide and is grounded and coupled to the top ground plane 720 byvias 731. These vias surround the position where antenna 721 ispositioned. Additional vias 731 (not shown for clarity) will couple thetop and bottom ground planes together across substrate 703. Solder ballsor bumps 711 surround an opening in the top ground plane 720 over theantenna 721, these solder bumps form a guide, similar to a waveguide,for directing the millimeter wave signals from the antenna upwards tothe package substrate (see FIG. 2 ). In additional alternativearrangements, instead of the coplanar waveguide 723, other transmissionline types can be used to couple the millimeter wave transmitterintegrated circuit 705 to an antenna. For example, microstrip,stripline, and other coplanar waveguides that are not grounded, or wherethe top ground plane is omitted, can be used.

FIG. 7B shows in detail a close up, plan view of the coplanar waveguidestructure 723. Note in FIG. 7B, conductor layers and vias are shownwhile the dielectric material is omitted for clarity. An antenna 721 ispositioned in a central opening in the top ground plane 720, the antenna721 is formed at the end of the coplanar waveguide 723 in a level ofmetal, and the vias 731 are arranged around the opening to act like awaveguide to guide the signals vertically towards the package substrate131 (not visible in FIG. 7A-7B, see FIG. 2A). Ground plane 726 isbeneath the antenna 721 and the coplanar waveguide 723, and covers thebottom of the opening in top ground plane 720. By using these featuresas millimeter wave guide structures, a cost effective microwave couplingbetween the millimeter wave transmitter module and the package substrateis created. The millimeter wave transmitter die 105 implements functionssuch as the “×12” multiplier in FIG. 1 , a power amplifier, sensors, andother circuit components needed to condition and transmit theinterrogation signal. Note that the receiver module 115 in FIG. 1 issymmetrical to the transmitter module 113 and the cross section of FIG.7A, and the plan view of the coplanar waveguide 713 of FIG. 7B, willapply to the millimeter wave receiver module 113 as well.

FIGS. 7C-7D are cross sectional views of the substrate 703 of themillimeter wave transmitter module 113, used to illustrate someadditional elements. In FIG. 7C, a side view of a portion of thecoplanar waveguide 723 with the antenna 721 is shown. Top ground plane720 has an opening 741 that corresponds to the opening 141 in thepackage substrate (not shown). A spacing distance “D” is shown betweenthe coplanar waveguide 723 and the bottom ground plane 726. The coplanarwaveguide 723 and the top and bottom ground planes 720, 726 areconductor layers and can be formed of copper, for example.

FIG. 7D is an additional cross sectional view of the coplanar waveguidelooking towards the antenna 721. Top ground plane 726 has an opening741, and bottom ground 720 is formed in a conductor layer beneath thecoplanar waveguide and antenna 721, which is a conductor layer, such aslevel 1, above the bottom ground plane layer 720. The distance “D” isdetermined to increase the efficiency of the transmission of themillimeter wave signals. For a frequency of about 121 GHz, for example,the wavelength (in air) is about 2.4 millimeters. The radiation lobesfrom the antenna 721 will be, initially, symmetric so that the signalradiates up towards the opening 741, and down towards the ground plane720. By choosing the distance D correctly, a constructive interferencesignal can be created by reflection so that an in-phase reflectionappears traveling upwards as oriented in FIGS. 3C, 3D and shifts theradiation lobe from a symmetric pattern to an asymmetric pattern withmost of the energy traveling upwards; in FIGS. 7C, 7D this isrepresented by the arrow. Assuming a reflection at the bottom plane 726shifts the signal by λ/2, the distance D should be selected to be λ/4,so that the total phase shift for a signal traveling down from theantenna 721 to the bottom ground 726 and back to the antenna 721 is(λ/4+λ/2+λ/4)=λ. When distance D is correctly determined, the reflectedsignal will add to the signal traveling upwards from the antenna 721 ina constructive interference pattern.

In the examples in FIGS. 7A-7D, coplanar waveguide is used as atransmission line o couple the millimeter wave integrated circuits toantennas. However, in alternative arrangements, microstrip or striplineconductors can be used. The top ground plane shown in the examples canbe omitted, or changed. The shape of the antenna shown is one example,other shapes can be used. High frequency simulation tools can verify theefficiency of a proposed design, and the design can be tuned todetermine the correct value for distance “D”, for example.

FIG. 8 illustrates, in a projection view, details of one example of aphysics cell 103. As seen in FIG. 8 , the physics cell 103 includes aserpentine gas chamber 111 that is lined with a conductor such as gold.A receiver iris 343 is at one end of the chamber, and a transmitter iris341 is at the other end of the gas chamber 111. The ends of the gaschamber 111 are positioned and aligned with the openings in the packagesubstrate 131 as shown above.

FIGS. 9A and 9B illustrate, in a side view and an end view, the module101 including the millimeter wave transmit and receive structures 133,135.

In FIG. 9A, the physics cell 103 is shown with chamber 111 on a deviceside surface, facing away from a system board side surface, of packagesubstrate 131. The transmit structure 133 includes the opening 141 inthe package substrate 131 and the solder balls 711 between the boardside surface of package substrate 131 and the transmitter substrate 703,while the receive structure 135 includes the opening 143 in the packagesubstrate 131 that is coupled to one end of the chamber 801 of physicscell 103, and the solder balls 711 that surround the waveguide (notvisible) on substrate 703. The millimeter wave transmitter module 113 isshown in part coupled tot eh package substrate 131 by the solder balls711. These elements are shown in an end view in FIG. 9B.

The transmit structure 133 includes the opening 141 in the packagesubstrate 131, the solder balls 711 on the millimeter wave transmittermodule 113, the coplanar waveguide and antenna structures shown in FIG.7B, and an iris for receiving signals at one end of the physics cell103. The receive structure 135 is symmetric, including an antenna at theopposite end of the physics cell 103, the opening 143 in the packagesubstrate 131, the solder balls 711 on the receive substrate of themillimeter wave receive module 115, and the coplanar waveguide andantenna for the receive module. By use of these elements, a costeffective, and temperature stable package is provided for the clockmodule including the millimeter wave transmitter, the millimeter wavereceiver, and the dipolar gas physics cell.

The arrangements reflect several design features selected to lower costof the packaged module. The use of the millimeter wave transmittermodule and millimeter wave receiver module (113, 115) requires a highfrequency high performance substrate. These substrates are relativelyexpensive and the arrangements are designed to reduce the size of thesemodules as much as possible. In contrast, the package substrate 131 canbe selected from a number of inexpensive, often used, widely available,low frequency materials. The size of the package substrate needs to beas large as needed to support the selected physics cell. For example,12×12 mm² to 25×25 mm² substrates can be used, the size increasing withthe length of the gas chamber in the physics cell.

Referring to FIG. 2A, it can be seen that the millimeter wave receiverstructure 135 and the millimeter wave transmitter structure 133 includeportions of the millimeter wave modules including the coplanar waveguideand antennas, the vias, the solder balls on the millimeter wave modules,the openings in the package substrate, and the iris openings in thephysics cell gas chamber. All of these elements are designed together,and work together to make efficient transmission of the millimeter wavesignals to the physics cell. Use of the openings in the packagesubstrate to transmit and receive the millimeter wave signals, withoutthe use of high frequency conductors and dielectrics in the packagesubstrate, reduces cost of the packaged clock module.

FIG. 10 illustrates in a flow diagram a method. In FIG. 10 , at step1001, millimeter wave signals are transmitted from an antenna throughair into openings to a dipolar gas in a physics cell mounted to apackage substrate. At step 1003, millimeter wave signals are receivedfrom the physics cell in a millimeter wave receiver module. As describedabove, this module is also mounted to the package substrate and receivessignals at an antenna over the air through an opening in the packagesubstrate. At step 1005, the receiver determines the strength of thesignals received, for example as shown in FIG. 1 , a square law detectorin a millimeter wave integrated circuit can be used in a receiver. Atstep 1009, the frequency of the transmitted signal is adjusted. Thisstep can be repeated to tune the circuitry if needed. At step 1011, afrequency corresponding to a quantum molecular rotational frequency isdetermined by detecting the frequency when the signal is absorbed by thephysics cell, as shown in FIG. 1 , a minimum amplitude may be detected.

Modifications are possible in the described arrangements, and otheralternative arrangements are possible within the scope of the claims.

What is claimed is:
 1. An apparatus, comprising: a package substratehaving a device side surface and a board side surface opposite thedevice side surface; a physics cell mounted on the device side surfaceof the package substrate and having a first end and a second end; afirst opening extending through the package substrate and lined with aconductor, the first opening aligned with the first end of the physicscell; a second opening extending through the package substrate and linedwith the conductor, the second opening aligned with the second end ofthe physics cell; a millimeter wave transmitter module mounted on theboard side surface of the package substrate, having a millimeter wavetransfer structure including a transmission line e coupled to an antennaconfigured to transmit millimeter wave signals aligned with the firstopening; and a millimeter wave receiver module mounted on the board sidesurface of the package substrate and having a millimeter wave transferstructure including a transmission line coupled to an antenna forreceiving millimeter wave signals from the physics cell, and alignedwith the second opening.
 2. The apparatus of claim 1, wherein thetransmission line in the millimeter wave transmitter module comprisesone selected from a coplanar waveguide, a grounded coplanar waveguide, amicrostrip, and a stripline.
 3. The apparatus of claim 1, wherein thephysics cell comprises a gas chamber containing a dipolar gas.
 4. Theapparatus of claim 3, wherein the dipolar gas is oxygen carbon sulfide(OCS) gas.
 5. The apparatus of claim 3, wherein the dipolar gas is a gasthat is one selected from oxygen carbon sulfide (OCS), water vapor,hydrogen chloride (HCL), hydrogen cyanide (HCN), and acetonitrile(CH₃CN).
 6. The apparatus of claim 4, wherein the OCS dipolar gas has aseries of discrete quantum rotational frequencies including a quantumrotational frequency of 121.62 GHz +/−10%.
 7. The apparatus of claim 1,wherein the first opening and the second opening are rectangular incross section and have a width and length of approximately 1 millimetersand approximately 2 millimeters, respectively.
 8. The apparatus of claim1, wherein the package substrate comprises a material that is oneselected from FR4, BT resin, ceramic, plastic, epoxy, fiberglass, glassreinforced epoxy laminate, paper based laminate, and semiconductorsubstrates.
 9. The apparatus of claim 1, wherein the package substratecomprises FR4, and the conductor lining the first and second openingscomprises copper or copper alloys.
 10. The apparatus of claim 1, whereinthe package substrate has a thickness between 1 millimeters and 5millimeters.
 11. The apparatus of claim 1, and further comprising ballgrid array terminals on the board side surface of the package substrate.12. The apparatus of claim 1, wherein the millimeter wave transmittermodule further comprises: a millimeter wave transmitter integratedcircuit mounted on a printed circuit board, the printed circuit boardhaving a top ground plane on a top surface facing the package substrate,a coplanar waveguide on a first conductor level within the printedcircuit board coupled to the millimeter wave transmitter integratedcircuit, a bottom ground plane formed on another conductor level withinthe printed circuit board positioned beneath the first conductor leveland the coplanar waveguide, and vias extending through dielectricmaterial of the printed circuit board and coupling the top ground planeand the bottom ground plane, the vias spaced from the coplanarwaveguide.
 13. The apparatus of claim 12, wherein the millimeter wavetransmitter module further comprises an opening in the top ground plane,the coplanar waveguide ending in an antenna positioned within theopening, the antenna configured to transfer millimeter wave signals fromthe coplanar waveguide through the opening in the top ground planethrough air, and to the first opening in the package substrate.
 14. Theapparatus of claim 13, wherein the millimeter wave transmitter module iscoupled to the package substrate by a plurality of solder balls spacedfrom one another that surround the opening in the top ground plane, thesolder balls attached to the bottom surface of the package substrate bysolder joints.
 15. The apparatus of claim 13, wherein the opening in thetop ground plane of the millimeter wave transmitter module is alignedwith the first opening in the package substrate.
 16. The apparatus ofclaim 1, wherein the millimeter wave receiver module further comprises:a receiver printed circuit board having an integrated circuit forreceiving millimeter wave signals mounted thereon, and having a topground plane on a top surface, a coplanar waveguide formed on aconductive layer within the printed circuit board, and a bottom groundplane on a conductive layer beneath the coplanar waveguide, the bottomground plane and the top ground plane coupled together by conductivevias that extend from the top ground plane to the bottom ground planethrough dielectric layers in the printed circuit board and spaced fromthe coplanar waveguide.
 17. The apparatus of claim 16, wherein thecoplanar waveguide is coupled between a receiver opening in the topground plane and the millimeter wave integrated circuit, the coplanarwaveguide having a probe in the receiver opening in the top ground planeof the printed circuit board that is aligned with the second opening inthe package substrate.
 18. The apparatus of claim 17, wherein thereceiver printed circuit board is mounted to the board side surface ofthe package substrate by solder balls.
 19. The apparatus of claim 17wherein the receiver printed circuit board is mounted to the board sidesurface of the package substrate by solder bumps.
 20. The apparatus ofclaim 19, wherein the receiver printed circuit board has at least threeconductor layers spaced by dielectric material.
 21. The apparatus ofclaim 1, wherein the millimeter wave signal has a frequency of 121.62GHz +/−10%.
 22. The apparatus of claim 3, wherein the dipolar gas is OCSgas, and the millimeter wave signal has a frequency that is one selectedfrom: 109.46 GHz +/−10%; 121.62 GHz +/−10%; and 133.78 GHz +/−10%.
 23. Asystem, comprising: a millimeter wave clock module comprising a physicscell with a gas chamber containing a dipolar oxygen carbon sulfide (OCS)gas with a quantum rotational frequency of greater than 109 GHz; aprocessor configured to control a clock generator coupled to themillimeter wave clock module, and to detect an amplitude signal from themillimeter wave clock module, the processor controlling the clockgenerator using the amplitude signal to lock a clock signal to thequantum rotational frequency of the millimeter wave clock module; and aclock output signal that the processor synchronizes to the clockgenerator, the clock output signal synchronized to the quantumrotational frequency of the millimeter wave clock module.
 24. The systemof claim 23, wherein the millimeter wave clock module further comprises:a physics cell comprising the dipolar gas in a gas chamber in asemiconductor substrate, the physics cell having a first end configuredto receive millimeter wave signals, and a second end configured totransmit millimeter wave signals; a package substrate having the physicscell mounted on a device side surface and having a board side surfaceopposite the device side surface; a first opening through the packagesubstrate aligned with the first end of the physics cell, the firstopening configured to transfer millimeter wave frequency signals to thegas chamber of the physics cell; a second opening through the packagesubstrate aligned with the second end of the physics cell, the secondopening configured to transfer millimeter wave frequency signals fromthe physics cell; a millimeter wave transmitter module mounted to thepackage substrate on the board side surface, the millimeter wavetransmitter module having a transmitter integrated circuit coupled to atransmission line and configured to transmit a millimeter wave frequencysignal to the physics cell; a millimeter waver receiver module mountedto the package substrate on the board side surface, the millimeter wavereceiver coupled to a transmission line and configured to receivemillimeter wave signals from the physics cell; and ball grid arrayterminals for the millimeter wave clock module on the board sidesurface.
 25. A method, comprising: transmitting millimeter wave signalsat an expected quantum rotational frequency from an antenna on amillimeter wave transmitter module through air to a physics cellcomprising a sealed gas chamber containing a dipolar gas; receiving themillimeter wave signals from the physics cell at an antenna in amillimeter wave receiver module; determining a signal strength of thereceived millimeter wave signals in the millimeter wave receiver module;adjusting the transmitted millimeter wave signals; and determining aquantum rotational frequency by detecting the frequency of thetransmitted millimeter wave signals when the signal energy received fromthe physics cell in the millimeter wave receiver module is at a minimumsignal energy.
 26. The method of claim 25, wherein the dipolar gas isone selected from one selected from oxygen carbon sulfide (OCS), watervapor, hydrogen chloride (HCL), hydrogen cyanide (HCN), and acetonitrile(CH₃CN).
 27. The method of claim 26, wherein the dipolar gas is OCS. 28.The method of claim 27, wherein the transmitted millimeter wave signalis at a frequency between 120 GHz and 122 GHz.
 29. The method of claim25, wherein the transmitted millimeter wave signal is 121.62 GHz +/−10%.30. The method of claim 25, wherein the transmitted millimeter wavesignal is at a frequency of between 109 GHz and 134 GHz.